On Wed, Mar 11, 2026 at 11:30:26AM -0400, Peter Xu wrote: > On Wed, Mar 11, 2026 at 12:02:05PM +0000, Daniel P. Berrangé wrote: > > On Mon, Mar 09, 2026 at 02:21:49PM -0400, Peter Xu wrote: > > > On Mon, Mar 09, 2026 at 05:51:29PM +0000, Daniel P. Berrangé wrote: > > > > On Mon, Mar 09, 2026 at 05:42:08PM +0000, Tejus GK wrote: > > > > > > > > > > > > > > > > On 9 Mar 2026, at 10:47 PM, Daniel P. Berrangé > > > > > > <[email protected]> wrote: > > > > > > > > > > > > !-------------------------------------------------------------------| > > > > > > CAUTION: External Email > > > > > > > > > > > > |-------------------------------------------------------------------! > > > > > > > > > > > > On Mon, Mar 09, 2026 at 12:59:44PM -0400, Peter Xu wrote: > > > > > >> On Mon, Mar 09, 2026 at 04:48:37PM +0000, Daniel P. Berrangé wrote: > > > > > >>>> @@ -881,8 +881,8 @@ static int > > > > > >>>> qio_channel_socket_flush_internal(QIOChannel *ioc, > > > > > >>>> sioc->zero_copy_sent += serr->ee_data - serr->ee_info + > > > > > >>>> 1; > > > > > >>>> > > > > > >>>> /* If any sendmsg() succeeded using zero copy, mark > > > > > >>>> zerocopy success */ > > > > > >>>> - if (serr->ee_code != SO_EE_CODE_ZEROCOPY_COPIED) { > > > > > >>>> - sioc->new_zero_copy_sent_success = true; > > > > > >>>> + if (serr->ee_code == SO_EE_CODE_ZEROCOPY_COPIED) { > > > > > >>>> + sioc->zero_copy_fallback++; > > > > > >>> > > > > > >>> ...this is counting the number of MSG_ERRQUEUE items, which is not > > > > > >>> the same as the number of IO requests. That's why we only used it > > > > > >>> as a boolean marker originally, rather than making it a counter. > > > > > >> > > > > > >> Would the logic still work and better than before? Say, it's a > > > > > >> counter of > > > > > >> "messages" rather than "IOs" then. > > > > > > > > > > > > IIUC it is a counter of processing notifications which is not > > > > > > directly > > > > > > correlated to any action by QEMU - neither bytes nor syscalls. > > > > > > > > > > Please correct me if I'm wrong about this, isn’t each notification an > > > > > information > > > > > about what happened to an individual IO? > > > > > > > > If userspace hasn't read a queued notification yet, the kernel will > > > > merge new notifications with the existing queued one. > > > > > > > > The line above your change > > > > > > > > serr->ee_data - serr->ee_info + 1; > > > > > > > > records how many notifications were merged, so we now how many > > > > syscalls were processed. > > > > > > > > If ee_code is SO_EE_CODE_ZEROCOPY_COPIED though it means at least > > > > one syscall resulted in a copy, but that doesn't imply that *all* > > > > syscalls resulted in a copy. > > > > > > > > AFAICT, it could be 1 out of a 1000 syscalls resulted in a copy, > > > > or it could be 1000 out of 1000 resulted in a copy. We don't know. > > > > > > > > IIUC the kernel's merging of notifications appears lossy wrt this > > > > information. It could be partially mitigated by doing a flush for > > > > notifications really really frequently but that feels like it would > > > > have its own downsides > > > > > > IMHO what this change does is removing the false negatives. > > > > > > Before this patch, if QEMU reports fallback=0, it doesn't mean all the > > > MSG_ZEROCOPY requests were all fulfilled by zerocopy. It's because we > > > justify it with one boolean over "a period of time" between two flushes, > > > we > > > set the boolean to TRUE as long as there is _one_ successful report of > > > MSG_ZEROCOPY. So even if every flush reports TRUE it only means "there is > > > at least one MSG_ZEROCOPY request that didn't fallback". It has no > > > implication of whether a fallback happened. > > > > > > Hence, before this v2 patch, there can be false negative reported by QEMU, > > > assuming there's no fallback (reflected in stats) but it actually > > > happened. > > > > > > After this patch, if QEMU reports fallback=0, it guarantees that _all_ > > > MSG_ZEROCOPY requests are fulfilled with zerocopy. It's because we > > > monitor > > > all messages and accumulate any fallback cases. Even if the messages can > > > be merged, when "fallback" shows anything non-zero would imply some > > > fallback happened. Here, the counter value doesn't really matter much > > > IMHO, as long as it becomes non-zero. > > > > AFAICT, the v1 of this patch was sufficient to address the original > > bug and maintain the current intended semantics of the migration > > counter. This v2 is mixing a bug fix with functional change in > > behaviour and I don't think the latter is justified. > > It's just that when it cannot report all fallback cases, I don't yet see > how it would help much even if we fix the previous behavior with v1.. > > OTOH, the new behavior will be deemed to have no issue on the problem v1 > was fixing. > > So IIUC v2's behavior is the one we want, and helps identify fallback > happened.
I don't consider v2 acceptable as the value its returning is an meaningless counter that doesn't correlate to any quantity that is used by QEMU, nor visible to users of QEMU. With regards, Daniel -- |: https://berrange.com ~~ https://hachyderm.io/@berrange :| |: https://libvirt.org ~~ https://entangle-photo.org :| |: https://pixelfed.art/berrange ~~ https://fstop138.berrange.com :|
